In the recent semiconductor processing technology, a challenge to higher integration of large-scale integrated circuits places an increasing demand for miniaturization of circuit patterns. There are increasing demands for further reduction in size of circuit-constructing wiring patterns and for miniaturization of contact hole patterns for cell-constructing inter-layer connections. As a consequence, in the manufacture of circuit pattern-written photomasks for use in the photolithography of forming such wiring patterns and contact hole patterns, a technique capable of accurately writing finer circuit patterns is needed to meet the miniaturization demand.
In order to form a higher accuracy photomask pattern on a photomask substrate, it is of first priority to form a high accuracy resist pattern on a photomask blank. Since the photolithography carries out reduction projection in actually processing semiconductor substrates, the photomask pattern has a size of about 4 times the actually necessary pattern size, but an accuracy which is not loosened accordingly. The photomask serving as an original is rather required to have an accuracy which is higher than the pattern accuracy following exposure.
Further, in the currently prevailing lithography, a circuit pattern to be written has a size far smaller than the wavelength of light used. If a photomask pattern which is a mere 4-time magnification of the circuit feature is used, a shape corresponding to the photomask pattern is not transferred to the resist film due to influences such as optical interference occurring in the actual photolithography operation. To mitigate these influences, in some cases, the photomask pattern must be designed to a shape which is more complex than the actual circuit pattern, i.e., a shape to which the so-called optical proximity correction (OPC) is applied. Thus, at the present, the lithography technology for obtaining photomask patterns also requires a higher accuracy processing method. The lithographic performance is sometimes represented by a maximum resolution. As to the resolution limit, the lithography involved in the photomask processing step is required to have a maximum resolution accuracy which is equal to or greater than the resolution limit necessary for the photolithography used in a semiconductor processing step using a photomask.
A photomask pattern is generally formed by forming a photoresist film on a photomask blank having a light-shielding film on a transparent substrate, writing a pattern using electron beam, and developing to form a resist pattern. Using the resulting resist pattern as an etch mask, the light-shielding film is etched into a light-shield pattern. In an attempt to miniaturize the light-shield pattern, if processing is carried out while maintaining the thickness of the resist film at the same level as in the art prior to the miniaturization, the ratio of film thickness to pattern width, known as aspect ratio, becomes higher. As a result, the resist pattern profile is degraded, preventing effective pattern transfer, and in some cases, there occurs resist pattern collapse or stripping. Therefore, the thickness of resist film must be reduced to enable miniaturization.
As to the light-shielding film material to be etched through the resist pattern as etch mask, a number of materials are known in the art. Among others, chromium compound films are used in practice because many teachings about etching are available and their processing has been established as the standard process. For example, a photomask blank having a light-shielding film composed of a chromium compound suited for ArF excimer laser lithography is disclosed in JP-A 2003-195479. Specifically a chromium compound film having a thickness of 50 to 77 nm is described.
A typical dry etching process for chromium base films such as chromium compound films is oxygen-containing chlorine gas dry etching, which has a certain etching ability relative to organic film. Thus, when etching is conducted through a thinner resist film in order to transfer a finer size pattern for the above-described reason, the resist film can be damaged during etching. It is then difficult to transfer the resist pattern accurately. To meet both the requirements of miniaturization and accuracy, it becomes necessary to investigate the light-shielding material again so as to facilitate the processing of light-shielding film, rather than the current trend relying solely on resist performance improvement.
For example, JP-A 2006-78807 discloses a light-shielding film including at least one layer of a material mainly containing silicon and a transition metal wherein an atomic ratio of silicon:metal is 4-15:1. The light-shielding film has improved light shielding function and ease of processing and is suited for the ArF lithography. Also JP-A 2007-241060 discloses a photomask blank comprising a light-shielding film containing silicon and a transition metal and a thin film of chromium base material as a hard mask film, with the advantage of high accuracy processing.